1. Field of the Invention
The present invention relates to a reflection optical system for forming a projection image and, more particularly, to a reflection optical system suitable for projection of a micropattern image such as of semiconductor device pattern.
2. Description of the Prior Art
A conventional reflection optical system of this type projects a mask pattern on a wafer pattern at an equal size. In a reflection type optical system disclosed in U.S. Pat. No. 3,748,015, a high-quality image is formed in an arcuate field of view by using concave and convex reflection surfaces, as in an optical system of Offner. Another reflection optical system using one concave reflection surface and one refracting member is proposed by J. Dyson, as per "Unit Magnification Optical System without Seidel aberrations", Journal of Optical Society of America, Vol. 49. P. 713, 1959.
In an Offner optical system, high performance is achieved by inserting a meniscus lens member, as exemplified in U.S. Pat. No. 4,331,390. In a Dyson optical system, various improvements including use of an achromatic lens are proposed by C. G. Wynne, "A Unit-Power Telescope for Projection Copying" in "Optical Instruments and Techniques" published by Oriel Press Limited and edited by J. H. Dickson, 1970.
However, these conventional systems are based on a unit magnification factor. When any of these conventional systems is used as an optical system in a projection exposure apparatus for semiconductor device manufacture, the size of the photomask must be the same as that of an integrated circuit, thus resulting in difficulty in the manufacture of photomasks. Another optical system is also proposed to project a reduction image using only a refracting system without a reflecting surface. This optical system comprises ten to 20 glass members. In this case, light absorption by the glass members is increased, resulting in inconvenience. In particular, when an exposure wavelength is shortened and far ultraviolet rays are used to satisfy the needs of further micropatterning of ICs, light absorption by the glass members becomes a critical problem. It is thus to be expected that patterning by the refracting system will be limited.
An advantage of a conventional optical system has been proposed by A. P. Grammatin in "Some Properties of Concentric Optical Systems", Optical Technology Vol. 38, No. 4, P. 210, 1970, wherein optical planes are monocentrically arranged, and an object point and an image point are formed in a plane perpendicular to an optical axis and including the center of the monocentric optical planes. The principle of operation of this optical system will be briefly described.
Since all the optical planes are monocentric, inclined angles of paraxial rays emitted from the monocentric center, i.e., from a point on an optical axis of the object surface, do not change their values except for their signs. A magnification factor is equal to a ratio of a refractive index of the object space to that of the image space. Similarly, the rays emitted from the object point along the optical axis do not change their inclined angles irrespective of the numerical aperture (NA). Therefore, the spherical aberration and the sine condition are strictly zero. When the sine condition is satisfied, coma is corrected in at least a third-order aberration region. Furthermore, when sagittal pencil and meridional pencil image surfaces are considered, the sagittal pencil image surface does not have a curvature of field for the same reason as for zero spherical aberration.